If Boxer A and Boxer B were fighting, and Boxer threw a punch that connected AFTER the bell had rang, and knocked boxer B out, what would happen and has anything like this happened before ?
yeah, it definitely has. I believe it happened in Tua-Rahman II, although most people thought the punch landed right at the bell. When it happens clearly after the bell and fighter B cannot continue, then fighter A is disqualified.
When Tyson fought Orlin Norris he knocked him out after the bell rang for the end of the first round. I can't remember all of the details but I believe it was ruled a no contest because Norris was complaining about his knee being injured during the knockdown. Tyson wasn't DQ'd though. As much as I know about Tyson, it pisses me off that I can't remember the details about this. Anyone who can shed some more light on the details of this fight, please do.
In most cases it will be a "No Contest". In the Rahman/Tua 2 fight - Rahman dropped Tua just a second after the bell - but the knock down didn't count. Even though it didn't count - it was nice. It came right at the end of the round - as they were exchanging punches. It's the only time I've ever seen Tua down.